1. FIELD OF THE INVENTION
The field of art to which this invention pertains is the solid bed adsorptive separation of dichlorotoluene (DCT) isomers. More specifically, the invention relates to a process for extracting the 2,5-isomer of dichlorotoluene (2,5-DCT) from mixtures of DCT isomers employing as the adsorbent L zeolites containing alkali metal cations.
2. BACKGROUND INFORMATION
The individual isomers of dichlorotoluene are useful in a variety of reactions, e.g., as intermediates for making pesticides, pharmaceuticals, peroxides, dyes, etc. The dichlorotoluene isomers are normally prepared by the non-catalytic nuclear chlorination of chlorotoluene in various solvents. The chlorinating agent comprises certain Lewis acid halides, e.g., hydrogen chloride in both liquid and vapor phase systems. The direct chlorination of ortho-chlorotoluene produces a mixture of 2,3-DCT, 2,4-DCT, 2,5-DCT and 2,6DCT while the direct chlorination of para-chlorotoluene produces 3,4-DCT and 2.4-DCT. The direct chlorination of toluene produces a mixture of 2,3-, 2,4-, 2,5-, 2,6-, and 3,4-DCT.
Separation of 2.5-DCT from the other isomers of dichlorotoluene by conventional distillation techniques is difficult due to the close boiling point range of these isomers. The following table shows the boiling points of the 2,4-DCT, 2,5-DCT, 3,5-DCT and 2,6-DCT isomers to be separated by less than 1.degree. C.
TABLE 1 ______________________________________ DCT Isomer Boiling Point (.degree.C.) ______________________________________ 2,3-DCT 208.3 3,4-DCT 208.9 2,4-DCT 201.1 2,5-DCT 201.8 2,6-DCT 200.6 3,5-DCT 201.2 ______________________________________
While it is possible to separate 2,3-DCT by distillation from the isomer mixture produced by the process of direct chlorination of o-chlorotoluene referred to above, the utilities duty of the overall process can be reduced substantially by our process by separating and recovering 2,5-DCT as the extract of a first adsorptive separation and distilling the raffinate containing 2,3-, 2,4- and 2,6-DCT to remove 2,3-DCT from the lowered volume rather than from the original feed. The remaining raffinate components can then be separated by a further adsorptive stage using the same adsorbent and conditions. This invention simplifies the separation procedure by providing an effective adsorptive separation method for recovering 2,5-DCT.
Crystalline alumina-silicates are commonly used in the separation art to perform adsorptive separations. The use of X or Y zeolites to separate individual isomers of dichlorotoluene is disclosed in U.S. Pat. No. 4,254,062. It is noted that 2,6-DCT is preferentially adsorbed onto the sodium or calcium form of X or Y zeolites, while the barium form or mixed barium-calcium form preferentially adsorb 2,4-DCT.
U.S. Pat. No. 4,774,371 discloses a static test for determining which dichlorotoluene isomers are adsorbed in the presence of other materials. For example, 2,6-dichlorotoluene is selectively adsorbed on X- or Y-type faujasite zeolites when a substituted benzene compound, preferably toluene, xylene, chlorotoluene or trimethylbenzene is present. Also, 3,5-DCT can be separated as the extract component when the substituted benzene compound is 1,2,4-trimethylbenzene or 2-chloro-p-xylene.
U.S. Pat. No. 4,766,262 and Japanese Public Disclosure No. 149636/87 dated July 3, 1987 disclose a selective adsorptive separation of dichlorotoluene isomers with a mordenite molecular sieve and an acid-treated mordenite type zeolite, respectively. In the former, 2,6-DCT is selectively adsorbed and recovered in the extract while in the latter 2,6-dichlorotoluene is non-adsorbed and recovered in the raffinate in a rejective-type separation. In 4,766,262, there appears to be little separation between the 2,5-, 2,4- and 3,4-isomers, although selectivity is noted as between the 2,6-DCT and 2,5-DCT isomers. In Examples 3 thru 5, and 9 and 10 (in vapor phase) of the patent, 2,5-DCT is selectively adsorbed with respect to 2,6-DCT, but it appears that 2,4-DCT is coextracted with the 2,5-DCT. In Examples 11 and 12, in liquid phase, 2,6-DCT is selectively adsorbed with respect to 2,5-DCT.
Japanese Public Disclosure No. 112034/86, dated May 30, 1986, and U.S. Pat. No. 4,777,306 disclose the separation of dichlorotoluene isomers on a ZSM-5 type zeolite. The adsorbed isomers can be desorbed with an eluent which may be a substituted and/or halogenated momocyclic or polycyclic aromatic compound, according to Japanese Public Disclosure 112034/86. In U.S. Pat. No. 4,777,306, 2,6-DCT is separated as the non-adsorbed isomer from the adsorbed isomers by a rejective separation process. In a comparative example in U.S. Pat. No. 4,777,306, it was disclosed that K-L zeolite had no adsorptive or separating effect.
Methods for forming the crystalline powders into agglomerates are also known and include the addition of an inorganic binder, generally a clay comprising a silicon dioxide and aluminum oxide, to a high purity zeolite powder in wet mixture. The blended clay zeolite mixture is extruded into cylindrical type pellets or formed into beads which are subsequently calcined in order to convert the clay to an amorphous binder of considerable mechanical strength. Clays of the kaolin type, water permeable organic polymers and silica may also be used as binders.
The invention herein can be practiced in fixed or moving adsorbent bed systems, but the preferred system for this separation is a countercurrent simulated moving bed system, such as described in Broughton U.S. Pat. No. 2,985,589, incorporated herein by reference. Cyclic advancement of the input and output streams can be accomplished by a manifolding system, which are also known, e.g., by rotary disc valves shown in U.S. Pat. Nos. 3,040,777 and 3,422,848. Equipment utilizing these principles are familiar, in sizes ranging from pilot plant scale (deRossett U.S. Pat. No. 3,706,812) to commercial scale in flow rates from a few cc per hour to many thousands of gallons per hour.
The functions and properties of adsorbents and desorbents in the chromatographic separation of liquid components are well-known, but for reference thereto, Zinnen et al. U.S. Pat. No. 4,642,397 is incorporated herein.
It has now been discovered that L-type zeolites exchanged with sodium, potassium or lithium cations or mixtures thereof at cation, exchange sites are suitable adsorbents for the separation of 2,5-DCT from other isomers of DCT, provided certain conditions in the chromatographic separation process are maintained. Two parameters that must be controlled in the process are the water concentration of the adsorbent and temperature of the process.